The present invention relates to data networks, and more particularly to code division multiple access (CDMA) networks.
In CDMA networks, a message is asynchronously transmitted as a plurality of code bits, typically several hundred to about one thousand, wherein the code bits include an information portion which contains a coded message and a header description portion containing codes indicating the terminals destined to receive the message. The large number of code bits requires a large bandwidth channel to maintain a reasonable data rate, while only a few terminals can use the channel if collisions between contending terminals are to be avoided. The requirements for a large bandwidth and few terminals in turn results in a low channel efficiency. CDMA (also known as spread spectrum multiple access, SSMA) has been utilized as a technique for asynchronous multipoint to multipoint communication in the presence of specular interference, jamming, etc. Until recently, CDMA was used mainly for military applications in which operational advantages due to spread spectrum coding take precedence over transmission efficiency. In the past few years, commercial application of CDMA has been increasing because it offers the possibility of interconnecting small earth stations or mobile terminals without timing synchronization, even in the presence of terrestrial interference.
Until recently, most CDMA systems were designed for quasi-continuous operation, in which each station is allocated a fixed fraction of channel resource and is assumed to operate nearly continuously. However, in many emerging applications, the individual demand for each terminal, such as in a point-of-sale terminal, is small and occurs in bursts so that operation of a packet network, in which resource use is proportional to station demands, becomes appropriate. In an "ALOHA" type system, erroneously received data packets are retransmitted with a random time delay. Consideration of CDMA packet networks for broadcast channels, including satellite and ground radio, leads to the conclusion that CDMA or random access type operation is preferred since CDMA reduces the probability of destructive interference due to multiuser transmissions. Since in CDMA, multiple transmissions can co-exist on the same channel without necessarily colliding, the throughput-delay characteristics will be different from conventional ALOHA, which does not use spread spectrum techniques.
A problem with current CDMA techniques is that spread spectrum coding generally requires a large expansion in bandwidth, so that net channel utilization is relatively low compared with unspread multiaccess methods. In order to achieve relatively high channel utilization, CDMA systems must be operated with a high channel traffic load. However, owing to the nature of random access (in which no control is exercised on the stations transmitting) unstable operation of the channel may result from too many transmissions being made in too short a time. Such an unstable operation implies that a backlogged retransmission mode has undesirably become a steady-state condition, which should be avoided. This is because the random access implied in CDMA systems result in traffic-dependent packet collisions with the unaltered retransmissions.
A practical CDMA system to be useful must incorporate a suitable mechanism to prevent such instability from occurring. Random access CDMA with fixed length packets and time slotting on the channel was discussed and analyzed in terms of throughput delay and stability by the present inventor in a paper entitled "Performance Analysis of Random Access Packet Switched Code Television Multiple Access Systems; IEEE Transactions on Communications, June 1981, pp. 895-901. However, a large proportion of practical CDMA Systems operate asynchronously. The prior art has not dealt with understanding the nature of the instability present in a general class of asynchronous random access SSMA packet networks with variable length message traffic. The present invention recognizes a need for understanding the nature of such instability and the need for a way to prevent such instability from occurring.
According to the present invention, a method for transmitting information comprises initially transmitting information in a code division multiple access channel. A procedure is then followed wherein a determination is made to see if the information has been correctly received at the transmitter and by inference, by other receivers, and, if not, the information is retransmitted with a selected time delay. This procedure is repeated using a different time delay each time until the information is determined to be correctly received. Time delays are selected at random but are selected to have an average value in accordance with the occupancy of the channel to provide a single stability mode to the system. The determination of whether these hasten correct reception may be inferred by the sending terminal based on it receiving back its own prior transmission free from collision.